In subduction zones, where one tectonic plate plunges under another, slow, imperceptible slips known as ‘slow earthquakes’ can trigger powerful tremors nearby. This has recently been demonstrated by researchers from CNRS, Université Grenoble Alpes and IRD, working with colleagues from the University of Mexico. Their study was published on 3 October 2016 in Nature Geoscience review.
‘Slow earthquakes’ were discovered around 20 years ago. They are imperceptible slips that last several weeks or months without causing seismic waves or damage. However, they can release as much energy as a 7.5-magnitude earthquake. Understanding these slow slips and their relations with regular earthquakes is thus fundamental when it comes to evaluating seismic risk. Now, for the first time, researchers have proven that a slow slip can trigger a regular earthquake. Researchers at the Institute of Earth Sciences (CNRS/Université Grenoble Alpes/IRD/Université Savoie Mont Blanc/IFSTTAR), together with colleagues from the University of Mexico, have demonstrated that the 7.3-magnitude earthquake that occurred in Papanoa on 18 April 2014 was the consequence of a slow slip that began two months earlier in the Acapulco region (Mexican state of Guerrero).
The geophysicists behind the discovery have been working for several years in this coastal zone where the oceanic plate of the Cocos Islands passes under the North American plate. This phenomenon, known as subduction, generally goes hand-in-hand with seismic activity since the two plates do not slide together seamlessly, but instead distort and accumulate energy, released during earthquakes. In the area studied, there had been no major earthquake since 1912, which gave rise to the name ‘seismic gap’. In addition, the installation of permanent GPS stations after 1997 has helped pinpoint slow slips: while the Cocos and North American plates approach one another at a speed of 5-6 cm/year, the gap area experiences periods of slip in the opposite direction every four years; these last around 6 months with movements of up to 15 cm.
Researchers have studied the GPS data and demonstrated that in the Guerrero seismic gap, slow slips release a portion of the pressure accumulated, which reduces the likelihood of a powerful earthquake. On the other hand, the slow slip that began in February 2014 transferred pressure to the neighbouring zone, which triggered the 7.3-magnitude earthquake near Papanoa on 18 April 2014.
This study helps us better understand the relationship between slow slips and regular earthquakes in a context of subduction. The impact of this research on society is significant since nearly 20 million people would be directly affected by devastation caused by a major earthquake on Mexico’s Pacific coast. This study highlights the increased risk of earthquakes during episodes of slow slip. More generally, it demonstrates the importance of studying signs of deformation in the days and weeks preceding significant earthquakes. A denser network of permanent GPS stations and the development of deep-sea GPS networks near earthquake initiation regions will allow for more precise characterisation of slow slips that precede regular earthquakes.